1. Field of the Invention
The present invention relates to an improved magnetic powder and a magnetic recording medium thereof. More particularly, it relates to magnetic powder having improved durability and a magnetic recording medium comprising the same.
2. Description of the Prior Art
A magnetic recording medium comprises a magnetic layer containing magnetic powder such as .gamma.-Fe.sub.2 O.sub.3, cobalt-containing epitaxial .gamma.-Fe.sub.2 O.sub.3, ferromagnetic metal, iron nitride or a hexagonal system ferrite. However, when the particle size of the magnetic powder is made smaller in order to decrease noise, durability of the magnetic recording medium is deteriorated due to low hardness of the magnetic powder. This is particularly noticeable when metal powder is used as the magnetic powder, since it has very low hardness.
To solve these problems, covering of the magnetic powder particles with Al.sub.2 O.sub.3 is proposed (cf. Japanese Patent Kokai Publication No. 134858/1977). According to this proposal, the covering is carried out by applying Al.sub.2 O.sub.3.nH.sub.2 O on particle surfaces of .alpha.-FeOOH powder, and thermally reducing the powder in a hydrogen gas stream to produce metal powder particles covered with Al.sub.2 O.sub.3. However, since the thermal treatment should be done at a temperature not higher than 1,000.degree. C. the on the surfaces remains in the spinel structure and has low hardness.
When .gamma.-Fe.sub.2 O.sub.3, cobalt-containing .gamma.-Fe.sub.2 O.sub.3 or hexagonal system ferrite is used as the magnetic powder, the same as described above can also apply.